An “orbital glass” of electron clouds

Can appear at low temperatures. In the modern picture of quantum mechanics, electron orbitals—which have various shapes including spheres and dumbbells—are thought of as clouds and represent the general region within which one may find an electron at any given time. A typical electronic transition, say between the degenerate states of a vertically oriented dumbbell and a horizontal one, occurs in femtoseconds. Now, scientists from Germany and Moldova have found evidence that these and other orbital processes can slow dramatically—by up to 14 orders of magnitude—for electrons in low-temperature, single-crystal FeCr₂S₄, a mineral with a relatively simple crystalline structure. The researchers found evidence that these frozen electron orbitals have glassy characteristics, including a residual entropy at 0 K, a hump in the crystal’s temperature-dependent specific heat, and broadened relaxation dynamics. In contrast to conventional glasses, a complete freeze of the electron clouds is precluded by quantum-mechanical tunneling: The clouds keep making transitions between different low-energy configurations without requiring thermal energy input. (R. Fichtl et al., Phys. Rev. Lett. 94, 027601, 2005.http://dx.doi.org/10.1103/PhysRevLett.94.027601 )